1. Field of the Invention
The present invention relates to a magnetic recording medium having a non-magnetic base and a magnetic layer coated thereon containing magnetizable particles dispersed in a resinous binder along with a lubricant.
2. Description of the Prior Art
As magnetic recording systems have been recently further developed and refined, various characteristics of the magnetic recording medium, such as the magnetic tape, magnetic sheet, magnetic disc or the like, have become increasingly important. For instance, in the case of a magnetic tape for recording a video signal, the so-called still (still picture) characteristic is of particular importance, while a so-called repeating travelling characteristic and anti-abrasion (wear resistance) characteristic are important in a magnetic tape for recording an audio signal and a magnetic tape for measuring.
Various binders, lubricants, additives and so on have been proposed to improve the above mentioned still characteristic, repeating travelling characteristic and wear-resistance characteristic. However, fully satisfactory results have not been realized. Further, it is desired to provide durable lubricant agents which do not deteriorate the electro-magnetic conversion characteristic, head wear, and viscous characteristic of paints in the form of which the magnetic layer is applied to the base. The primary functions of a lubricant are to smooth the travelling of a magnetic tape by reducing the coefficient of friction thereof when travelling at low speeds (2.4-38 cm/sec) and thereby to prevent the generation of Q-sounds and wow-flutters.
Lubricants which have been used for the magnetic layer have been exemplified as uniformly dispersed liquid substances, such as higher fatty acids (e.g. oleic acid, linoleic acid, ricinoleic acid, stearic acid, lauric acid, palmitic acid, capric acid, myristic acid, elaidic acid, stearolic acid and so on) and their fatty acid esters, one or more fatty acids of metallic salts, liquid paraffin, silicone oil, and so on, or powdered lubricating substances, such as, graphite, carbon, molybdenum disulfide, boron nitride, tungsten disulfide and powdered polytetrafluoroethylene, and mixtures thereof.
The melting point of the paraffin system and olefin system lubricants are in the range of -5.degree. C. to 50.degree. C. When these lubricants are used in place of olive oil, so-called "blooming" can result and the coefficient of friction is not reduced. "Blooming" occurs when additives contained in a magnetic layer of a magnetic recording medium diffuse to the surface of the magnetic layer and deposit thereon. Considerable amounts of deposits may be found even if the additives are in a liquid phase.
When a powder lubricant is used, the proportion of the magnetic layer formed of the magnetic powders or particles is reduced. Thus, the output signal from the magnetic tape is lowered and the uniform dispersion of the magnetic powders is disturbed. This disturbance results in "tape noise". Such a disturbed magnetic layer is unsuitable for a magnetic tape for recording a short-wave signal such as a video signal.
Japanese Patent Publication No. 28367/64 by Sony Corporation discloses a lubricant comprising a fatty acid ester of a monobasic fatty acid having 12 to 16 carbon atoms and a monohydric alcohol having 3 to 12 carbon atoms. Japanese Patent Publication No. 18064/66 by Sony Corporation discloses a lubricant comprising a fatty acid ester of a monobasic fatty acid having 18 carbon atoms and a lower or moderate monohydric alcohol. Japanese Patent Publication No. 18065/66 by Sony Corporation discloses a lubricant comprising a fatty acid ester of a monobasic fatty acid having 2 to 18 carbon atoms and a monohydric alcohol having more than 14 carbon atoms. A lubricant known from Japanese Patent Publication No. 1227/67 by Sony Corporation is myristic acid ester. A lubricant also known from Japanese Patent Publication No. 669/68 by Sony Corporation is a fatty acid ester of a monohydric alcohol having 27 carbon atoms and a monobasic fatty acid. Another lubricant known from Japanese Patent Publication No. 23889/68 by Fuji Photo Film Co. is a fatty acid ester having 21 to 23 carbon atoms in a molecule formed from a monobasic fatty acid having more than 17 carbon atoms and a monohydric alcohol. Other known lubricants are fatty acid esters of unsaturated alcohols disclosed in Japanese Patent Publication No. 12950/72 by Eastman Kodak Co., fluorine-containing fatty acid esters disclosed in Japanese Patent Publication No. 15002/73 by Sony Corporation, and fatty acid esters of saturated fatty acids having 12 to 22 carbon atoms and saturated alcohols having 1 to 8 carbon atoms disclosed in Japanese Patent Publication No. 25923/73 by Tokyo Denki Kagaku Kogyo Co.
Fatty acid esters of saturated fatty acids and saturated alcohols are generally good lubricants and the lubricating effect improves as the number of carbon atoms in a molecule increases. However, as the number of carbon atoms increases, the melting point of the fatty acid ester rises resulting in the blooming phenomenon, and, therefore, such fatty acid esters are not suitable for use as lubricants in a magnetic recording medium. For this reason, fatty acid esters having a lower melting point but larger number of carbon atoms in its molecule are desired as a lubricant for magnetic recording media.